- Research article
- Open Access
- Open Peer Review
Lack of association between PRNP 1368 polymorphism and Alzheimer's disease or vascular dementia
© Jeong et al; licensee BioMed Central Ltd. 2009
- Received: 29 August 2008
- Accepted: 08 April 2009
- Published: 08 April 2009
Polymorphisms of the prion protein gene (PRNP ) at codons 129 and 219 play an important role in the susceptibility to Creutzfeldt-Jakob disease (CJD), and might be associated with other neurodegenerative disorders. Several recent reports indicate that polymorphisms outside the coding region of PRNP modulate the expression of prion protein and are associated with sporadic CJD, although other studies failed to show an association. These reports involved the polymorphism PRNP 1368 which is located upstream from PRNP exon 1. In a case-controlled protocol, we assessed the possible association between the PRNP 1368 polymorphism and either Alzheimer's disease (AD) or vascular dementia (VaD).
To investigate whether the PRNP 1368 polymorphism is associated with the occurrence of AD or VaD in the Korean population, we compared the genotype, allele, and haplotype frequencies of the PRNP 1368 polymorphism in 152 AD patients and 192 VaD patients with frequencies in 268 healthy Koreans.
Results and conclusion
Significant differences in genotype, allele and haplotype frequencies of PRNP 1368 polymorphism were not observed between AD and normal controls. There were no significant differences in the genotype and allele frequencies of the PRNP 1368 polymorphism between Korean VaD patients and normal controls. However, in the haplotype analysis, haplotype Ht5 was significantly over-represented in Korean VaD patients. This was the first genetic association study of a polymorphism outside the coding region of PRNP in relation to AD and VaD.
- Prion Protein
- Vascular Dementia
- Prion Disease
- Korean Population
- PRNP Gene
Alzheimer's disease (AD), the most common cause of dementia in the aged population, is associated with progressive memory deterioration and disordered cognitive function resulting from a loss of cholinergic transmission and characterized neuropathologically by the presence of neurofibrillary tangles and amyloid plaques in the brain and clinically by gradual loss of memory. These changes may result from destructive processes involving the disruption of microtubule assembly and synaptic loss. Further neuronal damage and disease progression are consequences of this damage. Although the processes involved in AD could be triggered by many environmental factors, genetic studies have shown that in some cases mutations and polymorphisms of particular genes can confer susceptibility to the degenerative process. Several genes associated with AD have been identified, including amyloid precursor protein gene (APP ), presenilin-1 gene (PS1 ), presenilin-2 gene (PS2 ), and the apolipoprotein E gene (ApoE ) . AD and prion diseases, such as Creutzfeldt-Jakob disease (CJD), share a number of clinical, pathogenetic and pathological features. A structural hallmark of AD is amyloid-β peptide (Aβ) aggregates in extracellular amyloid deposits defined as senile plaques, while in CJD there is an accumulation of abnormal protease-resistant isoform (PrPres) in neurons and in extracellular amyloid-like aggregates. Aβ-positive senile plaques in AD brains commonly contain PrPC deposits [2–4] and incidental Aβ-positive senile plaques in prion diseases such as CJD may also be positive for PrPC .
Vascular dementia (VaD) is the second most common cause of dementia after AD. VaD is a clinical syndrome causing cognitive decline due to cerebrovascular lesions. Risk factors for VaD are age, sex, race, hypertension, smoking, diabetes mellitus, and hypercholesterolemia. However, there is no conclusive evidence for the association of genetic polymorphisms with VaD. VaD and prion diseases share some pathophysiological similarities, such as the occurrence of dementia.
Prion protein contains 253 amino acids encoded by prion protein gene (PRNP ), located on chromosome 20p12.3 in humans. PRNP plays an important role in conferring susceptibility or resistance to prion disease. A number of mutations in the open reading frame (ORF) are linked to the familial form of prion diseases [6, 7]. Polymorphisms at codons 129 or 219 of PRNP are susceptibility factors to sporadic CJD [8–11]. In several European populations, an association between the PRNP codon 129 polymorphism and AD was reported [12–15]. In contrast to these studies, other studies failed to detect a significant association between this polymorphism and AD [16–18], and in Asian populations, no association between the PRNP codons 129/219 polymorphisms and AD was reported . Recently, the polymorphism (PRNP 1368) in an upstream of PRNP exon 1 was found to be associated with sporadic CJD in British and German populations [20, 21], but this association was not seen in Dutch and Korean populations [22, 23]. This polymorphism was studied in other diseases in addition to sporadic CJD. In a British population, there was no association of PRNP 1368 polymorphism with frontotemporal lobar degeneration (FTLD) . Although PRNP 1368 polymorphism has been studied in sporadic CJD and FTLD patients, a case-controlled association study between the PRNP 1368 polymorphism and either AD or VaD has not been reported thus far.
In the present study, the purpose was to investigate the genotype and allele frequency of a polymorphism outside the coding region of PRNP in Korean AD and VaD patients and to determine the correlation between this polymorphism and the incidence of AD and VaD in the Korean population.
Characteristics of AD and VaD patients and controls
Number of subjects
Male, n (%)
Female, n (%)
Mean age at disease onset (years ± SD)
73.48 ± 8.00
71.95 ± 8.92
Mean age at blood collection (years ± SD)
71.17 ± 8.68
Genomic DNA was extracted from 200 μl blood using the QIAamp DNA blood mini kit (Qiagen, USA) following the supplier's instructions. Polymerase chain reaction (PCR) was performed with J-1 (GAGAAAACCTTGCGTCAGCA) and J-2 (AAGGTGCAGAAAAGATGGGC) primers. These primers were designed to amplify a 586 bp product in an upstream region of PRNP exon 1. The PCR reagents contained 50 pmole of each primer, 5 μl of 10 × Taq DNA polymerase buffer, 1.5 mM MgCl2, 0.2 mM of each dNTP mixtures, and 2.5 units of Taq DNA polymerase (Promega, USA). The PCR conditions were 94°C for 2 min to denature, and 35 cycles of 94°C for 45 sec, 56°C for 45 sec, and 72°C for 1 min 30 sec, and then 1 cycle of 72°C for 10 min to extend the reaction. The Perkin-Elmer Cetus DNA thermal cycler (Pekin-Elmer, USA) was used. Restriction cleavage sites were searched using Webcutter, ver. 2.0 (Carolina Biological Supply Co., USA). A 20 μl aliquot of purified PCR mixture was digested at 37°C for 1 h with 5 units of Pvu II (Invitrogen, USA). Restriction products were separated on a 1.5% agarose gel and visualized with ethidium bromide staining under UV light. The purification of PCR products for sequencing was done using a QIAquick gel extraction kit (Qiagen, USA). The PCR products were directly sequenced on an ABI 377 automatic sequencer using a Taq dideoxy terminator cycle sequencing kit (ABI, USA) and the same primers as indicated earlier in the standard conditions.
A χ2 test was used to determine whether the PRNP 1368 polymorphism was in Hardy-Weinberg equilibrium (HWE) in the Korean population. Odds ratios (OR) with 95% confidence interval (CI) and P -values were calculated by using the codominant model, controlling for age and sex as covariates. Differences in age of populations were analyzed using Student's t -test, and sex differences by using χ2 test. Haplotypes and their frequencies were inferred using the algorithm developed by Stephens et al. . Fisher's exact test was used to analyze differences in haplotype frequency between the normal population and patients with AD and VaD. The statistical powers were calculated using Statistical Power Calculator http://www.dssresearch.com/toolkit/spcalc/power_p2.asp.
Genotype and allele frequencies of the PRNP 1368 polymorphism in the normal population, AD patients, and VaD patients
(n = 268)
(n = 152)
(n = 192)
AD vs Control
VaD vs control
0.640 – 1.544
0.487 – 1.100
0.731 – 2.315
0.594 – 1.742
0.839 – 1.491
0.717 – 1.232
Haplotype frequency of three PRNP polymorphisms in the normal population, AD patients, and VaD patients
AD vs Control
VaD vs Control
AD vs VaD
In this study, we failed to detect a significant association between the PRNP 1368 polymorphism and the occurrence of either AD or VaD in the Korean population.
There is the possibility that the PRNP 1368 polymorphism is not functional with regard to affecting the level of PrPC. Another possibility is that a false negative result was obtained due to statistical powers. Data for AD and VaD patients showed a statistical power of 19.1% and 11.2%, respectively, at the Type I error rate of 0.05 compared with healthy controls. The statistical powers aren't high enough for ensuring that the PRNP 1368 polymorphism is not relevant to prion replication. However, in the haplotype analysis among 3 PRNP polymorphisms, haplotype ht5 was the only haplotype significantly associated with VaD (p = 0.013) (Table 3) and the genotype frequency of PRNP 1368 polymorphism in VaD patients was not in HWE. These results suggested some interaction among 3 PRNP polymorphisms in the determination of VaD risk and were needed for further evaluation of the association of PRNP 1368 polymorphism with VaD in other ethnic groups.
Although the exact function of the prion protein is not fully understood, it might be involved in the development and intensity of oxidative stress and, thereby, contribute to neurodegeneration. Thus, polymorphisms in the coding region of PRNP might influence other neurodegenerative disorders in addition to prion diseases. Many studies on a correlation between the PRNP codon 129 and AD in various populations have yielded contradictory results [12–19]. This controversial result may be due to the different sample size of the population analyzed, to a difference in frequency of PRNP genotypes between different ethnic groups , or even to a difference in age of onset. In our previous studies, we failed to detect a significant association between PRNP polymorphism at codons 129/219 and the risk for AD or VaD in the Korean population [30, 31].
Recently, there has been growing concern about several polymorphisms outside the ORF of PRNP , as there is evidence that levels of PRNP expression influence incubation time and the susceptibility to prion diseases. Polymorphisms in the PRNP promoter region may be associated with increased susceptibility of prion diseases in cattle and mice [32–34]. These PRNP promoter polymorphisms influence the PRNP gene expression level . Overexpression of PRNP in transgenic mice led to a decrease in incubation time, whereas PRNP knockout mice were resistant to prion disease after infection [36, 37]. Therefore, we suggest that the polymorphism of PRNP 1368, located in the promoter region, may influence the expression of the PRNP gene; the promoter polymorphisms of PRNP might also be associated with other neurodegenerative diseases. In previous studies, several polymorphisms were identified in intronic and upstream regions of human PRNP . The single nucleotide polymorphism (SNP) at position -101 (PRNP 12533) within the regulatory region of PRNP was associated with sporadic CJD in the British population , but not in samples derived from Dutch and German populations [20, 39].
Even though these results did not show a relationship between PRNP 1368 and AD or VaD, it would be useful to assess associations between AD/VaD and other PRNP polymorphisms in the promoter region, including the PRNP 12533 polymorphism.
PRNP 1368 polymorphism was not significantly associated with incidence of sporadic AD and VaD in Koreans. However, in the haplotype analysis among 3 PRNP polymorphisms, we observed a significant association between haplotype ht5 and VaD. Our report is the first association study of a polymorphism outside the coding region of PRNP with AD and VaD.
This work was supported by the Korea Research Foundation Grant funded by the Korean Government (MOEHRD) (KRF-2006-311-E00034).
- Bertram L, Tanzi RE: The genetic epidemiology of neurodegenerative disease. J Clin Invest. 2005, 115: 1449-1457. 10.1172/JCI24761.View ArticlePubMedPubMed CentralGoogle Scholar
- Esiri MM, Carter J, Ironside JW: Prion protein immunoreactivity in brain samples from an unselected autopsy population: findings in 200 consecutive cases. Neuropathol Appl Neurobiol. 2000, 26: 273-284. 10.1046/j.1365-2990.2000.00239.x.View ArticlePubMedGoogle Scholar
- Ferrer I, Blanco R, Carmona M, Puig B, Ribera R, Rey MJ, Ribalta T: Prion protein expression in senile plaques in Alzheimer's disease. Acta Neuropathologica. 2001, 101: 49-56.PubMedGoogle Scholar
- Kovács GG, Head MW, Hegyi I, Bunn TJ, Flicker H, Hainfellner JA, McCardle L, László L, Jarius C, Ironside JW, Budka H: Immunohistochemistry for the prion protein: comparison of different monoclonal antibodies in human prion disease subtypes. Brain Pathol. 2002, 12: 1-11.View ArticlePubMedGoogle Scholar
- Hainfellner JA, Wanschitz J, Jellinger K, Liberski PP, Gullotta F, Budka H: Coexistence of Alzheimer-type neuropathology in Creutzfeldt-Jakob disease. Acta Neuropathol. 1998, 96: 116-122. 10.1007/s004010050870.View ArticlePubMedGoogle Scholar
- Mead S: Prion disease genetics. Eur J Hum Genet. 2006, 14: 273-281. 10.1038/sj.ejhg.5201544.View ArticlePubMedGoogle Scholar
- Kovacs GG, Puopolo M, Ladogana A, Pocchiari M, Budka H, van Duijn C, Collins SJ, Boyd A, Giulivi A, Coulthart M, Delasnerie-Laupretre N, Brandel JP, Zerr I, Kretzschmar HA, de Pedro-Cuesta J, Calero-Lara M, Glatzel M, Aguzzi A, Bishop M, Knight R, Belay G, Will R, Mitrova E, EUROCJD: Genetic prion disease: the EUROCJD experience. Hum Genet. 2005, 118: 166-174. 10.1007/s00439-005-0020-1.View ArticlePubMedGoogle Scholar
- Petraroli R, Pocchiari M: Codon 219 polymorphism of PRNP in healthy Caucasians and Creutzfeldt-Jakob disease patients. Am J Hum Genet. 1996, 58: 888-889.PubMedPubMed CentralGoogle Scholar
- Jeong BH, Lee KH, Kim NH, Jin JK, Kim JI, Carp RI, Kim YS: Association of sporadic Creutzfeldt-Jakob disease with homozygous genotypes at PRNP codons 129 and 219 in the Korean population. Neurogenetics. 2005, 6: 229-232. 10.1007/s10048-005-0016-y.View ArticlePubMedGoogle Scholar
- Palmer MS, Dryden AJ, Hughes JT, Collinge J: Homozygous prion protein genotype predisposes to sporadic Creutzfeldt-Jakob disease. Nature. 1991, 352: 340-342. 10.1038/352340a0.View ArticlePubMedGoogle Scholar
- Shibuya S, Higuchi J, Shin RW, Tateishi J, Kitamoto T: Codon 219 Lys allele of PRNP is not found in sporadic Creutzfeldt-Jakob disease. Ann Neurol. 1998, 43: 826-828. 10.1002/ana.410430618.View ArticlePubMedGoogle Scholar
- Dermaut B, Croes EA, Rademakers R, Broeck Van den M, Cruts M, Hofman A, van Duijn CM, Van Broeckhoven C: PRNP Val129 homozygosity increases risk for early-onset Alzheimer's disease. Ann Neurol. 2003, 53: 409-412. 10.1002/ana.10507.View ArticlePubMedGoogle Scholar
- Gacia M, Safranow K, Styczyñska M, Jakubowska K, Pepłoñska B, Chodakowska-Zebrowska M, Przekop I, Słowik A, Golañska E, Hułas-Bigoszewska K, Chlubek D, Religa D, Zekanowski C, Barcikowska M: Prion protein gene M129 allele is a risk factor for Alzheimer's disease. J Neural Transm. 2006, 113: 1747-1751. 10.1007/s00702-006-0540-4.View ArticlePubMedGoogle Scholar
- Golanska E, Hulas-Bigoszewska K, Rutkiewicz E, Styczynska M, Peplonska B, Barcikowska M, Bratosiewicz-Wasik J, Liberski PP: Polymorphisms within the prion (PrP) and prion-like protein (Doppel) genes in AD. Neurology. 2004, 62: 313-315.View ArticlePubMedGoogle Scholar
- Riemenschneider M, Klopp N, Xiang W, Wagenpfeil S, Vollmert C, Muller U, Forstl H, Illig T, Kretzschmar H, Kurz A: Prion protein codon 129 polymorphism and risk of Alzheimer disease. Neurology. 2004, 63: 364-366.View ArticlePubMedGoogle Scholar
- Casadei VM, Ferri C, Calabrese E, Grimaldi LM, Franceschi M, Veglia F, Licastro F, Mariani C: Prion protein gene polymorphism and Alzheimer's disease: one modulatory trait of cognitive decline?. J Neurol Neurosurg Psychiatry. 2001, 71: 279-280. 10.1136/jnnp.71.2.279.View ArticlePubMedPubMed CentralGoogle Scholar
- Combarros O, Sánchez-Guerra M, Llorca J, Alvarez-Arcaya A, Berciano J, Peña N, Fernández-Viadero C: Polymorphism at codon 129 of the prion protein gene is not associated with sporadic AD. Neurology. 2000, 55: 593-595.View ArticlePubMedGoogle Scholar
- Del Bo R, Scarlato M, Ghezzi S, Martinelli-Boneschi F, Fenoglio C, Galimberti G, Galbiati S, Virgilio R, Galimberti D, Ferrarese C, Scarpini E, Bresolin N, Comi GP: Is M129V of PRNP gene associated with Alzheimer's disease? A case-control study and a meta-analysis. Neurobiol Aging. 2006, 27: 770e1-770.e5.View ArticleGoogle Scholar
- Ohkubo T, Sakasegawa Y, Asada T, Kinoshita T, Goto Y, Kimura H, Mizusawa H, Hachiya NS, Kaneko K: Absence of association between codon 129/219 polymorphisms of the prion protein gene and Alzheimer's disease in Japan. Ann Neurol. 2003, 54: 553-554. 10.1002/ana.10748.View ArticlePubMedGoogle Scholar
- Vollmert C, Windl O, Xiang W, Rosenberger A, Zerr I, Wichmann HE, Bickeböller H, Illig T, KORA group, Kretzschmar HA: Significant association of a M129V independent polymorphism in the 5' UTR of the PRNP gene with sporadic Creutzfeldt-Jakob disease in a large German case-control study. J Med Genet. 2006, 43: e53-10.1136/jmg.2006.040931.View ArticlePubMedPubMed CentralGoogle Scholar
- Mead S, Mahal SP, Beck J, Campbell T, Farrall M, Fisher E, Collinge J: Sporadic–but not variant–Creutzfeldt-Jakob disease is associated with polymorphisms upstream of PRNP exon 1. Am J Hum Genet. 2001, 69: 1225-1235. 10.1086/324710.View ArticlePubMedPubMed CentralGoogle Scholar
- Croes EA, Alizadeh BZ, Bertoli-Avella AM, Rademaker T, Vergeer-Drop J, Dermaut B, Houwing-Duistermaat JJ, Wientjens DP, Hofman A, Van Broeckhoven C, van Duijn CM: Polymorphisms in the prion protein gene and in the doppel gene increase susceptibility for Creutzfeldt-Jakob disease. Eur J Hum Genet. 2004, 12: 389-394. 10.1038/sj.ejhg.5201161.View ArticlePubMedGoogle Scholar
- Jeong BH, Lee KH, Lee YJ, Kim YH, Cho YS, Carp RI, Kim YS: PRNP 1368 polymorphism is not associated with sporadic Creutzfeldt-Jakob disease in the Korean population. Eur J Neurol. 2008, 15: 846-850. 10.1111/j.1468-1331.2008.02203.x.View ArticlePubMedGoogle Scholar
- Rohrer JD, Mead S, Omar R, Poulter M, Warren JD, Collinge J, Rossor MN: Prion protein (PRNP) genotypes in frontotemporal lobar degeneration syndromes. Ann Neurol. 2006, 60: 616-10.1002/ana.20931.View ArticlePubMedGoogle Scholar
- McKhann G, Drachman D, Folstein M, Katzman R, Price D, Stadlan EM: Clinical diagnosis of Alzheimer's disease: report of the NINCDS-ADRDA Work Group under the auspices of Department of Health and Human Services Task Force on Alzheimer's Disease. Neurology. 1984, 34: 939-944.View ArticlePubMedGoogle Scholar
- American Psychiatric Association, Diagnostic and Statistical Manual of Mental Disorders. 1994, (DSM-IV), Washington, DC, 143-147. FourthGoogle Scholar
- Roman GC, Tatemichi TK, Erkinjuntti T, Cummings JL, Masdeu JC, Garcia JH, Amaducci L, Orgogozo JM, Brun A, Hofman A, Moody DM, O'Brien MD, Yamaguchi T, Grafman J, Drayer BP, Bennett DA, Fisher M, Ogata J, Kokmen E, Bermejo F, Wolf PA, Gorelick PB, Bick KL, Pajeau AK, Bell MA, DeCarli C, Culebras A, Korczyn AD, Bogousslavsky J, Hartmann A, Scheinberg P: Vascular dementia: diagnostic criteria for research studies. Report of the NINDS-AIREN International Workshop. Neurology. 1993, 43: 250-260.View ArticlePubMedGoogle Scholar
- Stephens M, Smith NJ, Donnelly P: A new statistical method for haplotype reconstruction from population data. Am J Hum Genet. 2001, 68: 978-989. 10.1086/319501.View ArticlePubMedPubMed CentralGoogle Scholar
- Jeong BH, Nam JH, Lee YJ, Lee KH, Jang MK, Carp RI, Lee HD, Ju YR, Jo SA, Park KY, Kim YS: Polymorphisms of the prion protein gene (PRNP) in a Korean population. J Hum Genet. 2004, 49: 319-324. 10.1007/s10038-004-0150-7.View ArticlePubMedGoogle Scholar
- Jeong BH, Lee KH, Jeong YE, Hwang KA, Lee YJ, Carp RI, Ju YR, Kim YS: Polymorphisms at codons 129 and 219 of the prion protein gene (PRNP) are not associated with sporadic Alzheimer's disease in the Korean population. Eur J Neurol. 2007, 14: 621-626. 10.1111/j.1468-1331.2007.01786.x.View ArticlePubMedGoogle Scholar
- Jeong BH, Na HR, Bae JC, Lee KH, Lee YJ, Kim NH, Song JH, Carp RI, Kim YS: Absence of association between codon 129 and 219 polymorphisms of the prion protein gene and vascular dementia. Dement Geriatr Cogn Disord. 2007, 24: 86-90. 10.1159/000103913.View ArticlePubMedGoogle Scholar
- Jeong BH, Lee YJ, Kim NH, Carp RI, Kim YS: Genotype distribution of the prion protein gene (PRNP) promoter polymorphisms in Korean cattle. Genome. 2006, 49: 1539-1544. 10.1139/G06-110.View ArticlePubMedGoogle Scholar
- Kashkevich K, Humeny A, Ziegler U, Groschup MH, Nicken P, Leeb T, Fischer C, Becker CM, Schiebel K: Functional relevance of DNA polymorphisms within the promoter region of the prion protein gene and their association to BSE infection. FASEB J. 2007, 21: 1547-1555. 10.1096/fj.06-7522com.View ArticlePubMedGoogle Scholar
- Jeong BH, Sohn HJ, Lee JO, Kim NH, Kim JI, Lee SY, Cho IS, Joo YS, Carp RI, Kim YS: Polymorphisms of the prion protein gene (PRNP) in Hanwoo (Bos taurus coreanae) and Holstein cattle. Genes Genet Syst. 2005, 80: 303-308. 10.1266/ggs.80.303.View ArticlePubMedGoogle Scholar
- Sander P, Hamann H, Drögemüller C, Kashkevich K, Schiebel K, Leeb T: Bovine prion protein gene (PRNP) promoter polymorphisms modulate PRNP expression and may be responsible for differences in bovine spongiform encephalopathy susceptibility. J Biol Chem. 2005, 280: 37408-37414. 10.1074/jbc.M506361200.View ArticlePubMedGoogle Scholar
- Bueler H, Aguzzi A, Sailer A, Greiner RA, Autenried P, Aguet M, Weissmann C: Mice devoid of PrP are resistant to scrapie. Cell. 1993, 73: 1339-1347. 10.1016/0092-8674(93)90360-3.View ArticlePubMedGoogle Scholar
- Westaway D, Mirenda CA, Foster D, Zebarjadian Y, Scott M, Torchia M, Yang SL, Serban H, DeArmond SJ, Ebeling C, Prusiner SB, Carlson GA: Paradoxical shortening of scrapie incubation times by expression of prion protein transgenes derived from long incubation period mice. Neuron. 1991, 7: 59-68. 10.1016/0896-6273(91)90074-A.View ArticlePubMedGoogle Scholar
- McCormack JE, Baybutt HN, Everington D, Will RG, Ironside JW, Manson JC: PRNP contains both intronic and upstream regulatory regions that may influence susceptibility to Creutzfeldt-Jakob Disease. Gene. 2002, 288: 139-146. 10.1016/S0378-1119(02)00466-3.View ArticlePubMedGoogle Scholar
- Bratosiewicz-Wasik J, Liberski PP, Golanska E, Jansen GH, Wasik TJ: Regulatory sequences of the PRNP gene influence susceptibility to sporadic Creutzfeldt-Jakob disease. Neurosci Lett. 2007, 411: 163-167. 10.1016/j.neulet.2006.08.001.View ArticlePubMedGoogle Scholar
- The pre-publication history for this paper can be accessed here:http://www.biomedcentral.com/1471-2350/10/32/prepub
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